Continuous casting with in-line stream degassing

ABSTRACT

In a continuous casting process, a tundish feeds a molten metal stream through a &#39;&#39;&#39;&#39;vacuum&#39;&#39;&#39;&#39; chamber in contact with the mold. Discrete gas bubbles are injected into the molten metal, prior to its exit through a nozzle orifice in the tundish, so that the bubbles are entrained with the metal entering the &#39;&#39;&#39;&#39;vacuum&#39;&#39;&#39;&#39; chamber. The expansion of the gas bubbles in the chamber breaks the metal stream into a spray of fine droplets with the concurrent removal of dissolved gases from the metal. The degassed metal is then fed into the top of the mold and is continuously cast.

United States Patent ()lsson et al.

CONTINUOUS CASTING WITH IN-LINE STREAM DEGASSING Inventors: Robert G. Olsson, Edgewood Boro;

Ethem T. Turkdogan, Pittsburgh, both of Pa.

United States Steel Corporation, Pittsburgh, Pa.

Filed: Apr. 24, 1972 Appl. No.: 247,036

Related US. Application Data Continuation-impart of Ser. No. 148,551, June 1, 1971.

Assignee:

US. Cl. 75/49, 75/59, 164/64 Int. Cl. C21c 7/10, B22d 11/10 Field of Search 75/49, 59; 164/64 References Cited UNITED STATES PATENTS 6/1958 Rozian 164/64 7/1959 Lorenz ..75/49 2,997,386 8/1961 Feichtinger 75/59 1,323,583 12/1919 Earnshaw... 75/49 3,367.396 2/1968 Sickbert 75/49 3,042,510 7/1962 Armbruster... 75/49 3,320,053 5/1967 Lehman 75/49 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney-Arthur J. Greif 5 7] ABSTRACT In a continuous casting process, a tundish feeds a molten metal stream through a vacuum chamber in contact with the mold. Discrete gas bubbles are injected into the molten metal, prior to its exit through a nozzle orifice in the tundish, so that the bubbles are entrained with the metal entering the vacuum" chamber. The expansion of the gas bubbles in the chamber breaks the metal stream into a spray of fine droplets with the concurrent removal of dissolved gases from the metal. The degassed metal is then fed into the top of the mold and is continuously cast.

9 Claims, 1 Drawing Figure CONTINUOUS CASTING WITH IN-LINE STREAM DEGASSING This invention is a continuation-in-part of application Ser. No. 148,551, filed June 1, 1971.

This invention is directed to an improved method for the degassing of molten metal in a continuous casting process, and is particularly related to the vacuum degassing and vacuum carbon-deoxidation of steel during such a casting process.

Vacuum degassing, especially in its adaptation to steel making, has long been recognized as important to the production of high quality product. Although the advantages of vacuum processing are well known, it has not been widely employed in the continuous casting of steel slabs, due to the problems inherent in the presently available vacuum degassing techniques.

In the D-H process, a long nozzle at the lower end of an evacuated vessel is immersed in a ladle of molten steel and the vessel is repeatedly lowered and raised. Atmospheric pressure causes the steel to first rise into the vacuum chamber (during the lowering of the vessel) and then run back into the ladel (when the vessel is raised). By repeating this operation for a number of cycles, the entire contents of the ladle can be degassed. The intensive mixing resulting from this repeated cycling and the non-contaminating atmosphere within the vessel, advantageously permits the addition of a variety of alloys and deoxidizers, however, the extent of degassing achieved is poorer than in the stream method described below. More importantly, this method is not readily adapted to the in-line processing required for continuous casting, thereby necessitating a subsequent transfer of the degassed metal in the ladle to the tun dish prior to casting. Unless a number of special precautions are taken during this transfer, oxygen pick-up and recontamination will readily occur.

In the R-H method, the evacuated vessel is equipped with two long nozzles. One of these nozzles is provided with a means for the injection of an inert gas, and both nozzles are lowered into the molten steel in a ladle. Injection of the inert gas effects a decrease in the density of the metal and causes the metal in that nozzle to rise into the vacuum chamber. The metal entering the chamber is degassed and then returned to the ladle via the second nozzle. The circulation of the metal which is achieved thereby, is continued until the desired degree (with the limits obtainable by this process) of degassing is attained. Here again, while substantial mixing is achieved, the poor degree of degassing and the inadaptability to in-line processing result in a product which is not much cleaner than that achieved without vacuum treatment.

In the stream degassing process, the molten steel is teemed through an orifice in the bottom of the containing ladle, into an evacuated chamber and is received by another ladle set within the chamber. This method will, in general, provide more effective degassing and is readily adopted to in-line processing (see, for example, U. S. Pat. Nos. 3,125,440 and 3,540,515). However, since there is no recirculation as in the above processes, little mixing of alloy and deoxidizer additions is attainable thereby. Additionally, the extent of degassing achieved in all three of the above processes is seriously dependent on a. the degree of vacuum achieved in the chamber, and

b. the level of dissolved gases in the steel.

Thus, if the vacuum is poor, e.g., a pressure of about 0.001 atm. or higher, degassing is generally ineffective. Similarly, if chemical deoxidation is employed prior to vacuum treatment, the lowered level of dissolved gases will make the removal of other gases, e.g., H N less effective.

The instant process, which is a modification of the stream degassing process, overcomes these difficulties since it is (l) readily adapted to in-line processing, (2) is not dependent on the level of dissolved gas, and (3) will provide significantly more effective degassing over a wide range of vacuums. Thus, even at pressures which would be considered exceedingly poor or soft vacuums, e.g., up to 0.25 atmospheres, the instant process will provide gas removal, superior to prior art processes employing hard" vacuums, e.g., below 0.00l atmospheres.

In the instant process, a vacuum chamber is placed above and is sealably connected to the continuous casting unit. Molten metal is poured from the transfer ladle into the tundish at a rate necessary to meet the needs of the caster. Non-deleterious gas bubbles, e.g., Argon in the case of steel, are injected into the steel just above the nozzle orifice in the bottom of the tundish. The gas bubbles are thereby entrained in the metal stream entering the vaccum chamber. As the metal enters the chamber, the gas bubbles rapidly expand and break the stream into a spray of very fine droplets, markedly increasing the surface area of metal which is exposed to the lowered pressure in the chamber. As the spray droplets descend, a large percentage of the dissolved gases, e.g., oxygen and hydrogen, are removed by vac uum degassing and vacuum carbon deoxidation. The droplets may then impinge upon a refractory cone and the resulting liquid drains into the top of the mold where it is cast as in conventional practice. The use of a refractory cone is preferred since it will protect the interior of the chamber and the top of the mold from splashes and buildup of sprayed metal. In a further embodiment, this cone is cooled (by external means) to control the temperature of the steel entering the mold and/or to protect the cone itself, with a layer of solidified metal.

It is therefore an object of this invention to provide an in-line continuous process for the vacuum degassing of molten metal.

It is another object of this invention to provide improved degassing performance at vacuums significantly poorer" than was heretofore possible.

It is a further object to achieve effective degassing of killed steels.

Another object of this invention is to prevent the reoxidation or other contamination of the metal between the degassing and casting steps.

These and other objects and advantages will be more apparent when read in conjunction with the appended claims and the following detailed description, in which:

The FIGURE is a representation of a system which may be employed for the practice of the invention;

The FIGURE illustrates a preferred embodiment of this invention, which is particularly adapted to the degassing of molten steel. The metal to be degassed in transfer ladle l is teemed into the tundish 2. Nondeleterious gas bubbles, e.g., Argon or other inert gas, are injected into the metal prior to its entrance into nozzle orifice 3. While the gas bubbles may be injected at any point within the tundish, they will be utilized more efficiently if they are injected at a point in close proximity to the orifice (as depicted) since;

a. the pressure of the metal (and therefore the pressure within the bubbles) at such a depth will be greater, and

b. a greater portion of the injected bubbles will be entrained within the increment of liquid entering the nozzle.

The length of nozzle 3 (the term nozzle is meant to include a knife edge orifice, i.e., a device for converting the pressure and potential energy of a fluid into kinetic energy) is preferably kept short, in order that the passage of metal with its entrained bubbles will be sufficiently rapid so as to prevent the bubbles from expanding to a substantial extent prior to their entrance into vacuum chamber 4. To prevent such undesirable expansion, it is therefore preferred that the liquid be urged through the nozzle with sufficient force so that the residence time of the liquid within the nozzle is:

where t residence time in seconds, and

P pressure in atmospheres, of the liquid at the point of bubble injection The relative size of the nozzle opening is less critical. However, to achieve the desired, sudden accelaration of the liquid as it passes through the nozzle orifice, the cross-sectional area of the opening should preferably be less than one half the cross-sectional area of the tundish. In this manner, the bubbles entrained within the increment of metal are suddenly taken from the high pressure region of the tundish to the low pressure region of the chamber where bubble expansion significantly enhances the radial spread of the metal stream and the ensuing spray of fine droplets.

in this regard, it should be noted that the pressure, P within the chamber need not be less than one atmosphere to achieve degassing. Thus, it is only essential that the pressure within the chamber be substantially less than P i.e., the pressure existing within the metal in the containment vessel prior to its entrance into the nozzle orifice. Therefore, it is the ratio of pressures P,/P which is controlling and not the absolute pressure P within the chamber. However, it is desirable that P be maintained at a pressure below about 0.25 atmospheres, since operation at such a reduced pressure will provide both a cleaner steel product and will reduce the tendency of the casting to bulge after leaving the mold.

A variety of devices may be employed for effecting the injection of discrete gas bubbles. Two such devices are shown, i.e., a raised channel 5 to carry a gas line or a separate lance 6. In the preferred embodiment, both are utilized, with the gas line serving as the primary source and the lance serving as a stand-by. It is essential that the diameter of the injected bubbles be substantially smaller than the orifice opening, so as to provide a metal matrix with a discontinuous gas phase entrained therein, i.e., discrete gas bubbles. A closure 7, such as the sliding gate shown, on the tundish nozzle is employed to establish or preserve the vacuum at the beginning or end of a cast.

A variety of different chambers for maintaining a zone of reduced pressure may be employed. A preferred system as shown in the FIGURE, employs a bellows arrangement to isolate the movement of the conventional oscillating molo 8. Similar isolation of mold movement could be achieved by use of such other ex-' pedients as sliding joints or O-rings.

From pilot scale experiments, described in application Ser. No. 148,551, it was seen that chamber pressures as high as 0.25 atmospheres could be employed (for effective removal of oxygen), depending on (a) the rate of gas injection, and (b) the desired quantity of CO removed. Thus, if optimum rates of gas injection are employed, e.g., where the ratio of gas volume to liquid metal is about 2 to 4:1, it is only necessary that the partial pressure of CO in the chamber be less than the corresponding level from the desired product. For example, consider the case where ppm of oxygen is to be removed from the steel to yield a final partial pressure of CO above the steel of 1/10 atm. The removal of this quantity of oxygen is equivalent to the generation of approximately 4.5 ft of CO per ton of steel. lf argon is added at the rate of 10 ft /ton of steel, the CO woulo be sufficiently dilute to permit operation at a vacuum of A atmosphere. However, it may readily be seen, that if 200 ppm of oxygen were to be removed, a somewhat lower pressure would be required in the chamber. Similarly, the effective removal of H i.e., for example, to below 2 ppm would also necessitate the use of lower chamber pressures. in any practical case, however, the required vacuum would be considerably softer than any prior process. lt may therefore be seen, that in utilizing the instant process, there is no need to construct the chamber to withstand the forces which would be created by the much lower pressures of the prior art. Similarly, the lines to the vacuum pump will be significantly smaller and may be constructed of flexible material so as to facilitate removal of the chamber from the mold. Further economies may also be realized, since only a moderate pump capacity is required, e.g., a single stage steam ejection unit (not shown).

As the metal spray disperses within the chamber, the droplets are preferably caused to strike a refractory cone 9 which serves to contain and collect the spray and protect the chamber surfaces. The leeward surfaces of the cone are desirably cooled, e.g., by coils 10 to both protect the cone and to control the temperature of the steel emptying into the mold. Other than in the provision of a seal to chamber 4, e.g., as by O-rings no change is necessary in the conventionally employed caster molos.

* All gas volumes are at standard temperature and pressure, (STP). However, the controlled atmosphere above the mold eliminates the need for the protective slag covers and/or the shrouding which are normally required to protect against further contamination of the steel.

In addition to the above benefits, the reduced pressure above the mold entrance effects a significant reduction in the ferrostatic head in the casting itself, thereby reducing the tendency of the casting to bulge or break out after leaving the mold. Thus, even when the pressure in the chamber is as high as 0.25 atmospheres, the effective ferrostatic head of molten steel will be reduced by about 3% feet. Of course, even greater reduction in the ferrostatic head (hence, greater reduction in break-out tendency) will be realized at lower vacuum chamber pressures.

Since the disintegration of the metal stream is not dependent on saturating or dissolving a gas within the liquid, any non-deleterious" gas may be injected for the purpose of spray formation. Thus, in addition to argon or other gases chemically classified as inert," other gases may be employed which do not react detrimen tally with the metal being treated.

We claim: 1. A method for the in-line degassing of molten metal during the continuous casting thereof, which comprises teeming the metal from the vessel in which it is contained, through a nozzle and into a zone maintained at a pressure, P substantially lower than the pressure of the metal within said vessel, the crosssectional area of said nozzle being less than 0.5 the cross-sectional area of said vessel;

injecting a non-deleterious gas into said metal prior to its passage through said nozzle, at a point wherein the metal is at a pressure, P,, substantially higher than P said gas being injected at a rate by which spaced apart bubbles are formed, said bubbles being of a diameter substantially smaller than the diameter of the nozzle opening; and

urging said metal through said nozzle with sufficient force so that the residence time of the liquid in said nozzle is not greater than that defined by the equation:

where t= residence time in seconds and P is the pressure, in atmospheres, of the metal at said point of gas injection; whereby the resultant expansion of the bubbles in said low pressure zone enhances the radial expansion and disintegration of the metal into fine droplets;

combining said droplets to form a degassed metal pool and directing said metal pool through a mold for casting into a desired shape.

2. The method of claim 11, wherein the pressure, P of said zone is a soft vacuum.

3. The method of claim 1, wherein said metal is steel and said non-deleterious gas is an inert gas.

4. The method of claim 3 wherein P is less than 0.25 atmospheres.

5. The method of claim 4, wherein said point of injection is substantially proximate said nozzle so as to cause a major portion of said bubbles to be entrained within the increment of steel entering said nozzle.

6. The method of claim 5, wherein said mold is sealably connected to said lower pressure region so that the surface of steel in said mold is only in contact with said reduced pressure.

7. The method of claim 6, wherein the combining of said droplets is effected by causing said droplets to impinge upon a refractory surface.

8. The method of claim 7, wherein said refractory surface is externally cooled.

9. The method of claim 8, wherein said refractory surface is in the form of the frustrum of an inverted cone, whereby said steel pool is caused to drain into the top of said mold. 

2. The method of claim 11, wherein the pressure, P2, of said zone is a soft vacuum.
 3. The method of claim 1, wherein said metal is steel and said non-deleterious gas is an inert gas.
 4. The method of claim 3 wherein P2 is less than 0.25 atmospheres.
 5. The method of claim 4, wherein said point of injection is substantially proximate said nozzle so as to cause a major portion of said bubbles to be entrained within the increment of steel entering said nozzle.
 6. The method of claim 5, wherein said mold is sealably connected to said lower pressure region so that the surface of steel in said mold is only in contact with said reduced pressure.
 7. The method of claim 6, wherein the combining of said droplets is effected by causing said droplets to impinge upon a refractory surface.
 8. The method of claim 7, wherein said refractory surface is externally cooled.
 9. The method of claim 8, wherein said refractory surface is in the form of the frustrum of an inverted cone, whereby said steel pool is caused to drain into the top of said mold. 